Question

Under constant pressure, a system of gases is sealed in a cylinder and then allowed to expand. What can you conclude about the work associated with this change?
A. Work is positive and is done to the system.
B. Work is positive and is done by the system.
C. Work is negative and is done to the system.
D. Work is negative and is done by the system.

Answer 1

Well work is under constant pressure, AKA isobaric conditions \[W = -p~\Delta V\]

Answer 2

we know that the volume increases right? do Vb-Va >0 our change in volume would be greater than zero. that would make work positive or negative based on the equation above?

Answer 3

I do not really understand @Photon336

Answer 4

So work is force times distance. think of a person, pushing a box right? you apply a force which is the pushing over a distance. you're doing work.

Answer 5

@fifib

Now when we talk about a cylinder with a gas in it.

the distance is more along the lines of the change in volume.

Pressure is a force. it's defined as Force applied over an area.

Answer 6

Okay that makes sense. So the work is positive? The molecules are doing work @Photon336

Answer 7

yeah, so go back to the first formula,
'
\[\Delta V = V_{2}-V_{1}\]

Answer 8

what does expansion mean to you and how does it apply to our problem?

Answer 9

@fifib

Answer 10

Oh it is done by the system huh? if i'm wrong that is embarrassing

Answer 11

If I remember correctly, it's the work done by the system

Answer 12

You know that volume increases, so the final volume is bigger than the initial one, so what do you think you would have to do?

Answer 13

with what?

Answer 14

So you know that the volume expands right? that means our final volume is bigger than our initial one so when we take the difference it's going to be positive.

Answer 15

Postie means work done on the system :P

Answer 16

positive*

Answer 17

Hey @fifib do u still need help ?

Answer 18

@Rushwr YES

Answer 19

\[W = P . \Delta V\]
W= work done
P = Pressure
Delta V = Volume difference

Answer 20

Volume difference = final volume - initial volume right?

Answer 21

i honestly do not even understand the entire chapter @Rushwr

Answer 22

When it is expansion, the volume expands and the final volume increases therefore the volume difference becomes a positive value.
When Delta V is positive work done (W) becomes positive.
W = P . Delta V
Since the pressure is constant when Delta V becomes positive the work done becomes positive.

When Work done is positive we say that work is done by the system.
When the work is negative we say that the work is done on the system

Answer 23

That is part u need for the question.
But let's take the situation where contraction occur.
When contraction happen the final volume of the system decreases than the initial volume
Therefore the volume difference becomes a negative value.
(Think the initial volume was 5dm^3 and the final volume is 2dm^3 , then the volume difference is 2-5 = -3 )
I hope you understood the example.
Then the work done becomes negative according the equation therefore we say the work is done on the system.

Answer 24

Do you get it now ?

Answer 25

Since I didn't give an example for expansion let's take it here.
If the final volume of a system is 10dm^3 and the initial volume of the system before work was done was 6dm^3
So now you find the volume difference
Delta V = final volume - initial volume = 10 -6 = 4dm^3
No negative value therefore the volume difference becomes a positive one leading the work done to be positive.

Answer 26

That made so much sense @Rushwr

Answer 27

I hope you are getting it now :)

Answer 28

@Rushwr welcome back! @fifib hey you still need help?

Answer 29

@Photon336 I'm good :) thank you

Answer 30

No problem

Answer 31

hhaaaaaiiii @Photon336